This invention relates to a process for determining the activity of .alpha.-amylase in a sample using a modified oligosaccharide as a substrate and a process for producing such a modified oligosaccharide.
Heretofore, various processes for determining the activity of .alpha.-amylase using a modified oligosaccharide as a substrate have been proposed. For example, U.S. Pat. No. 4,649,108 to Blair discloses a method of measuring the amount of .alpha.-amylase in liquid sample comprising the steps of providing an oligosaccharide substrate for .alpha.-amylase, said substrate containing at least 3 glucose units, the reducing-end glucose residue (unit) being bonded, via a bond cleavable by .alpha.- or .beta.-glucosidase, to a label which exhibits an optically measurable change upon cleavage of said bond, and the non-reducing-end (terminal) glucose residue being bonded to a blocking group which inhibits cleavage by exo-enzymes of the bond between said non-reducing-end glucose redidue and the adjacent glucose residue; contacting said sample with said oligosaccharide substrate and with a first added exo-enzyme capable of cleaving the bond between said reducing-end glucose residue and said label, and a second added exo-enzyme capable of cleaving bonds between glucose units, to form a mixture comprising said substrate, said first exo-enzyme, and said second exo-enzyme; and measuring said optically measurable change. According to said U.S. patent, the blocking groups can be carboxylic acid esters, phosphate esters, sulfonate esters, ethers (such as benzyl, silyl, and triphenylmethyl), monosaccharides other than .alpha.-1,4 linked glucose, and acetal or ketal blocking groups such as benzylidene (column 4 lines 37-67). But the ester blocking groups are unstable since they are easily hydrolyzed in an aqueous solution. Further, when the size of the blocking group becomes larger as in the case of toluenesulfonyl, methanesulfonyl, silyl or triphenylmethyl, the solubility in water is undesirably lowered. In addition, the introduction of a blocking group into the non-reducing-end glucose residue has many problems, and the ester and ether groups as mentioned above cannot be introduced into the non-reducing-end glucose residue by the process disclosed at from column 4 line 63 to column 7 line 58 of said U.S. patent. Therefore, according to said U.S. patent, preferably the substrate has eight or fewer glucose units, and most preferably has six or seven, and preferred blocking substituents are acetals or ketals, e.g. benzylidene (column 2 lines 4-7).
When the most preferred oligosaccharide according to said U.S. patent, that is, an oligosaccharide having 6 or 7 glucose units and benzylidene as the blocking group, is used as a substrate for measuring .alpha.-amylase, there are many problems in that the hydrolysis rate by .alpha.-amylase is slow, various kinds of hydrolyzed products are produced due to many positions to be hydrolyzed, the reaction for liberating the color producing group becomes multiple and complicated due to many hydrolyzed products, the amount of coupling enzymes to be used becomes larger, and thus the reliability for the measurement becomes insufficient. The improvement of these disadvantages has long been desired.